To clarify a bit, basically the filtering by code doesn't seem to work at all in here. It returns all the wiki metrics when I specify what code I want. Running the devserver, the following filters do work:

http://localhost:8000/en-US/api/v1/wikimetrics/?code=active_contributors

http://localhost:8000/api/v1/wikimetrics/?code=percent_localized_top20

etc